Chromatography Forum

One of the two pumps is giving me grief as it gives pressure oscillations at low flows. It stabilizes at around 600 psi and varies only +-3 psi, but at around 400 psi it jumps up and down 50 psi. The oscillations show up on the UV detector when I start mixing a little bit of the flow from this "suspicious" pump into an established flow for the other, "reliable" pump (A). Oscillations are highly periodical, and the frequency changes proportionally to the % of flow from the "suspicious" pump (B). The frequency is ~5 Hz at 10% pump B, 90% pump A at 1 ml/min total. Switching mobile phases between pumps (ACN and water) did not change the observed behavior. Pumps as Shimadzu LC-10ADvp, both have new check valves (ASI) and piston seals. The "reliable" pump does not oscillate at any flows or back pressures. Any ideas?

I haven't worked with that specific pump, but it sounds as though a "hydraulic" pulse dampener is malfunctioning on the "suspicious" pump. The pulse dampener is sort of like a shock absorber and it does require a certain amount of back pressure to function properly. If it's reasonably accessible, you might try swapping the pulse dampener between the units and see if the problem follows.

An interesting point. As far as I can see, there is no pulse dampener on either pump. The lines from the pumps go into a 25 ul mixer, but it probably does not much to dampen pressure oscillations. I've heard that check valves may need some substantial back pressure to function properly, but since the check valves are the same on both pumps, I thought that the difference in behavior is coming from the pumps. I guess my easiest (and cheapest) solution may be to install a flow restrictor made out of a capillary which will raise the back pressure above the threshold, and see if this fixes the problem. Thanks.

irrespective of whether the check-valves are new, the symptoms are consistent with incomplete closing of a check-valve at lower back-pressure. When you have the problem, is the pump delivering the expected volume?

Check valve problems should be easy to confirm/eliminate: just swap the check valves between the two pumps.

A clogged mobile phase inlet filter may cause this .
You may also check the compressibility factor in the pump menu , it is around 0.5 default for water.

Purge the pump after replacing the filter two or three times .

Thanks everyone for all good suggestions. Flow restrictor helped, but when I removed it, the problem did not come back. Compressibility factor was set correctly (PULSE=48 for water). Actually, this was a good point because it was not set up right for the better working pump . The problem is gone, at least for now. Both pumps are very stable and deliver an accurate volume of solvents. Let us wait for it to come back.

After getting read of the problem and living without it for a while, it seems like the root cause was an air bubble.